"We needed this ROV to be able to go deeper and conduct the same research that the submersibles are doing in shallower water.

"The ROV is absolutely critical for mission success."

Image:The ROV was described as 'critical for mission success'

Image:One of the team can't hide his delight at the successful rescue

I was diving in one of the mission's manned submersibles on Tuesday when the accident happened, and found the ROV lying on its side 100m below the surface.

We nudged it with the sub's robotic arm, but didn't have enough battery power left after a six-hour dive to attempt recovery.

A further effort to attach a float failed and a third was thwarted by strong currents.

If the ROV is still working it will be used for Deep Ocean Live, a pioneering live broadcast by Sky News from 300 metres below the surface.

:: Sky News will broadcast live from 300m down in the Indian Ocean on 18, 19 and 20 March. The series will examine the impact of climate change and plastic pollution. It includes the deepest-ever live news programme from submersibles.

:: Sky's Ocean Rescue campaign encourages people to reduce their single-use plastics. You can find out more about the campaign and how to get involved at www.skyoceanrescue.com.